This contract will support the murine retrovirus genetics and pathogenesis programs of the Viral Oncology and Virology and Cellular Immunology Sections of LIP. The contract effort has 2 main functions: 1) To maintain breeding and small scale production colonies of congenic mouse strains developed for use in the study of host- and virus-related aspects of murine leukemia virus (MuLV) infection. The majority of strains are NFS (inbred NIH Swiss) carrying one or more esotropic murine leukemia virus (MuLV) loci derived from the AKR/N, C58/LN, and C3H/Fg mouse strains. The contract responsibility includes selected typing for expression of MuLV (by preparing tail biopsy extracts and carrying out tissue culture assays by XC plaque testing), setting up mating combinations of appropriate mice at proper intervals, monitoring of health of breeding stock, selective holding for long term observation for disease, shipment of sick animals to LIP and LMM investigators, and provision of specified animals for viral pathogenesis studies. 2) To house and regularly monitor for neoplastic or other disease various strains of mice inoculated with MuLVs. Mice are evaluated for general health, and specifically examined for splenomegaly and lymphadenopathy, respiratory distress, and signs of neurologic disease. There are occasional requirements for XC testing and test bleeding. Sick mice are identified and sent to LIP for autopsy.